Pathologists have often drawn attention to the supposed favorable prognostic significance of intensive leucocytic infiltration in human tumors. Despite numerous such reports the infiltrates have been rarely quantitated morphometrically or classified cytologically or functionally. We propose to compare in the line 10 guinea pig hepatoma the leucocytic infiltrate of progressing tumors with that of tumors undergoing vaccine-induced regression. We shall identify those features of the leucocytic infiltrate of regressing tumors which are predictive of the favorable outcome. The results should indicate which parameters of the infiltrate in human tumors are worthy of intensive investigation and ultimately predict a favorable prognosis.